The widespread use of optical devices in the communications industry has created a significant need for high performance electro-optical devices which are compatible with conventional semiconductor devices and fabrication techniques. One particularly acute need is for a phase modulating optical wave guide.
Thin film light guiding devices hold great promise for future use in conjunction with other optical devices and conventional integrated semiconductor devices. Ultimately, the thin films and coupling devices may be deposited on an integrated circuit chip, light emitting diodes, photodetectors, and the like, to produce integrated electro-optical circuits.
While prior art devices have been developed for guiding light beams, these developments do not include a practical thin film phase modulator. Generally, prior art wave guide devices include an input coupler, a thin film path for transmitting the light beam, and a substrate. The light beam typically enters the film through an optical coupler, such as a prism coupler or grating coupler, at an angle of incidence greater than the critical angle, thereby causing the light to travel through the thin film by multiple internal reflections between the outer surface of the thin film and the surface of the thin film adjacent the substrate. After the light has proceeded through the thin film, it is coupled to a utilization device such as a photoelectric cell. Output coupling is accomplished either by direct contact with the thin film or by the use of an optical coupler.
While attempts have been made to develop thin film bulk effect modulators using the Pockels effect or the Kerr effect, little success has yet been achieved in making these devices compatible with integrated circuits. For example, a thin film wave guide using the Kerr effect in Yttrium-gallium-scandium-iron garnet has been developed which can vary the polarization of a laser beam, but the device suffers from several distinct disadvantages which limit its practical employment. For example, it is very difficult to fabricate a garnet wafer on the same chip with conventional semiconductor devices. In addition, the amount of voltage required to change the light phase is too high (in the order of 10.sup. 3 - 10.sup.5 volts per cm.) to make the device compatible with conventional semiconductor circuitry. Moreover, the speed with which the beam can be modulated is also quite low. Generally, modulation signals cannot exceed 80 MHz. Other known bulk effect light modulators, such as Pockles effect devices, suffer substantially the same deficiencies.
A gallium phosphide wave guide device operating on the electro-optic effect has been recently developed. This device, while enjoying a number of advantages over previous bulk effect devices, nonetheless suffers from practical disadvantages which limit its use in integrated circuits. For examples, the gallium phosphide requires that the light beam be directed into the wave guide at a precise angle. Improper orientation results in unacceptable losses. Moreover, losses are very high even when the light is properly directed. In addition, the device requires a relatively high voltage (in the order of 15 volts) for activation. Although this voltage is significantly below the voltage required by bulk effect devices, it is not easily interfaced with other integrated circuit components which usually operate with a supply of about 5 volts.